This invention relates to a mobile radiotelephone communication system in which communication channels are reused in different parts of an overall mobile service area. The invention relates, in particular, to a technique for suppressing cochannel interference sufficiently to improve perceived voice signal quality in received signals.
In cellular channel reuse mobile radiotelephone systems, such as that described in the entire January, 1979, issue of the Bell System Technical Journal (BSTJ), it is known that frequency channels can be reused in different parts of a service area if the parts are sufficiently spaced to prevent intolerable cochannel interference between adjacent reuse locations. This reuse distance D, is determined in the form of a compromise between the size of cells which are employed in the system and the number of channel sets of a given block of radio channels, as well as the maximum number of channel sets required to handle maximum cell traffic volume in any given cell. Prior cellular systems that have been implemented using frequency modulation (FM) techniques, as in the aforementioned BSTJ, at least in part, because FM is usually less demanding than in amplitude modulation (AM) in the realms of frequency stability, circuit linearity, and insensitivity to fading. This situation prevailed even though channel bandwidth required for AM single sideband (SSB) is usually substantially less than is required for FM. Advances in the state of the art have mitigated somewhat the linearity aspect. Similarly, the fading and stability problems have been reduced by proposals such as those in the copending U.S.A. patent applications entitled "Single Sideband Receiver With Pilot-Based Feed Forward Correction For Motion-Induced Distortion," U.S. Pat. No. 4,313,211, for K. W. Leland, and "Frequency Stabilization Circuit For a Local Oscillator," Ser. No. 06/097,422, filed Nov. 26, 1979, for K. W. Leland and N. R. Sollenberger. Both such applications are assigned to the same assignee as the present application. Relief in these areas has made cochannel interference a more significant concern, and the present invention is directed to a technique for improving perceived voice signal quality in the presence of cochannel interference.
Attempts have been made in the prior art to resolve the dilemma, given a fixed number of available channels, of needing, on one hand, a relatively few channel sets per cell reuse pattern so that there will be a sufficiently large number of channels in each cell for good traffic handling capability and needing, on the other hand, many channel sets per cell reuse pattern in order to realize a relatively large reuse pattern which affords substantial geographical separation between cochannel antenna sites. An R. H. Frenkiel U.S. Pat. No. 4,144,411 shows one example of an FM cellular system utilizing cochannel site spacing for suppressing cochannel interference. Otherwise, prior art techniques have usually involved the employment of marker tones to facilitate the task of distinguishing among transmissions received at a single point from a plurality of closest cochannel stations. Such a marker tone use is sometimes called "coloring," e.g., as in color coding wires of a cable. In these systems, if an incorrect marker tone, sometimes called a supervisory audible tone (SAT), is detected, the station receiver is quieted; and a call in progress is terminated if the incorrect marker tone persists beyond a predetermined interval.
An E. J. Addeo U.S. Pat. No. 4,025,853 illustrates one technique for utilizing the SAT signal in an FM cellular system for suppressing cochannel interference. In a paper entitled "AGC, AFC Tone Select Circuits for Narrow Band Mobile Radio" by B. B. Lusignan, presented at the International Telecommunications Exposition (Intelcom, 1979), Dallas, Tex., February, 1979 (copies hand distributed), and in another paper by the same author entitled "Spectrum Efficiency of Single Side-band Radio with Amplitude Compandors," presented in the March, 1979, IEEE Vehicular Technology Group Conference in Arlington Heights, Ill. (copies hand-distributed), a system is disclosed in which a marker tone is modulated onto a pilot signal adjacent to the voice band in a channel of an AM SSB system; and if the wrong tone is detected in the absence of voice transmissions in a received signal, the station receiver is squelched to relieve the listener of the need to listen to messages from others utilizing the same channel at different stations.
In yet another SSB system described by R. W. Gibson and R. Wells in "The Potential of SSB for Land Mobile Radio," IEEE Vehicular Technology Conference Record, Mar. 27-30, 1979, pages 90-94, companding is used to reduce the relative effect of cochannel interference during pauses in speech in a desired channel. However, such an arrangement has no significant effect in terms of improving desired channel voice quality during speech times.